That depends heavily on circumstances including how often the one is reading, their interest level, whether or not they are taking detailed notes, how often they are interrupted and for how long (like 5 minutes or more like 6 months?), &c.
As a data point, the first time I read the sequences (about 13 years ago, before they were in their current format), it took me about 2 months to get all the way through. For reference: I am
fairly extremely distractible, was working a full time job, had fairly demanding family responsibilities, found the material reasonably accessible and very interesting, and was not taking notes of any kind. I hardly expect the sequences were taking up more than half of my reading time, so I probably could have done it in a month or less if I'd been trying to make a project of it.
I also expect that it would take me substantially longer today. I've established a habit of taking copious notes on texts like that, and I now have a small child who demands a large portion of my attention when I'm at home. I am not less distractible, still work full time (though the schedule has changed such that I have more days off but my days on are basically toast as far as getting anything else done), and I suspect that I would have a harder time focusing on material that I've mostly already read. I'd estimate 3-6 months, but wouldn't be terribly surprised if it went longer. If I get around to actually rereading, I'll update with the start and end dates.